Hydrogen is an indispensable component for many petroleum and chemical processes as well as increasingly in other areas such as a fuel for Fuel Cells. Refineries in the petroleum industry, and methanol, cyclohexane, and ammonia plants in the chemical industry consume considerable quantities of hydrogen during processes for the production of gasoline and fertilizers and other chemical products. As environmental regulations demand cleaner, renewable and non-polluting processes and products, most of the hydrogen balances at petroleum refineries are becoming negative. As laws mandate lower aromatics in gasoline and diesel fuels, H2 is now consumed in aromatic saturation and thus, less H2 is available as a by-product. At the same time, H2 consumption is increasing in hydro-treating units in the refineries because many of these same laws require lower sulfur levels in fuels.
Hydrogen can be obtained as a byproduct in the catalytic reforming of naphtha. In particular, significant amounts of hydrogen can be obtained during dehydrocyclization of naphtha in selective processes such as the Aromax™ process. Hydrogen is also obtained by steam reforming methane or mixture of hydrocarbons, a reaction which produces synthesis gas which comprises hydrogen, carbon dioxide and carbon monoxide (CO). Synthesis gas represents one of the most important feedstocks of the chemical and petroleum industries. It is used to synthesize basic chemicals, such as methanol or oxyaldehydes, as well as for the production of ammonia and pure hydrogen. However, synthesis gas produced by the steam reforming of hydrocarbons does not meet the requirements for further use in some processes because the CO/H2 ratio is too high. Therefore it is industrial practice to reduce or adjust the CO content in the syngas by conversion with steam in what is often referred to as the water-gas shift (WGS) reaction.
To improve H2 yield and also the operating efficiency of carbon monoxide conversion, the water-gas shift reaction is extensively used in commercial hydrogen or ammonia plants. The reaction can be described as:CO+H2O⇄CO2+H2 The water-gas shift reaction is usually divided into a high temperature process and a low temperature process. The high temperature process is generally carried out at temperatures within the range of between about 350 and about 400 degrees C. The low temperature water-gas shift reaction typically takes place between about 180 and about −240 degrees C.
While lower temperatures favor more complete carbon monoxide conversion, higher temperatures allow recovery of the heat of reaction at a sufficient temperature level to generate high pressure steam. For maximum efficiency and economy of operation, many plants contain a high temperature reaction unit for bulk carbon monoxide conversion and heat recovery and a low temperature reaction unit for final carbon monoxide conversion.
Chromium-promoted iron catalysts have been used in the high temperature process at temperatures above about 350 degrees C. to reduce the CO content to about 3–4% (see, for example, D. S. Newsom, Catal. Rev., 21, p. 275 (1980)). As is known from the literature (see for example, H. Topsoe and M. Boudart, J. Catal., 31, p. 346 (1973)), the chromium oxide promoter combines two functions. It serves to enhance catalytic activity and acts as a heat stabilizer, i.e., it increases the heat stability of magnetite, the active form of the catalyst, and prevents unduly rapid deactivation.
Unfortunately, when chromium is used, especially in hexavalent form, expenditures must be incurred to guarantee worker safety both during production and later handling of the catalyst, and health hazards cannot be fully ruled out despite considerable effort. In addition, the spent catalyst ultimately poses a hazard to man and the environment and must be disposed of with allowance for the government regulations relating to toxic waste.
Catalysts used for the water-gas shift reaction at low temperature (or so-called low temperature shift reaction) in industry generally contain copper oxide, zinc oxide and aluminum oxide. Because these catalysts operate at relatively low temperature, they generate equilibrium carbon monoxide concentrations of less than 0.3% in the exit gas stream over an active low temperature shift catalyst. However, carbon monoxide conversion and hydrogen yield gradually decreases during normal operations as a result of deactivation of the catalyst. Deactivation can be caused by sintering and poisoning such as by traces of chloride and sulfur compounds in the feed and the hydrothermal environment of the reaction. The rate of the hydrothermal deactivation, in particular, is dependent on reaction conditions such as the temperature, the steam to gas ratio and composition of the feed gas mixture, and the formulation and manufacturing process of the catalyst.
Although copper is physically and physicochemically stabilized by both zinc oxide and aluminum oxide and attempts of further stabilization of the catalyst have been made as taught by prior art, sintering of copper crystallite is still thought to be a significant cause for deactivation/aging of the catalyst, especially when there are very low concentrations of poisons in the feed. For example, the copper crystallite size of a fresh catalyst can range from 30–100 angstroms in contrast with 100–1,000 angstroms for a discharged spent catalyst. Low temperature shift catalysts thus need to be improved with regard to activity and stability.
Another use for hydrogen that is becoming increasingly important is as a feedstock to a fuel cell to generate electricity. The Proton Exchange Membrane (PEM) fuel cell is one of the most promising fuel cell designs and PEM fuel cells are already commercially available in limited applications. PEM fuel cells as well as several other fuel cell designs currently in development require hydrogen as a feedstock along with oxygen. Processes being considered to supply the needed hydrogen include Steam Reforming, Partial Oxidation (POX), Autothermal Reforming, and variations thereof. Most such processes for hydrogen generation also produce Carbon Monoxide (CO). Yet many Fuel Cells, in particular PEM fuel cells, cannot tolerate CO and in fact can be poisoned by small amounts of CO. The water-gas shift reaction can be used to generate additional hydrogen and convert the CO into the more inert CO2. Many fuel cells including PEM fuel cells can tolerate CO2 although it can act as a diluent.
As mentioned above one of the most common methods for the hydrogen production using hydrocarbons is the steam reforming process or variations thereof. The main process step involves the reaction of steam with a hydrocarbon over a catalyst at about 800° C. to produce hydrogen and carbon oxides. It is typically followed by several additional steps to remove impurities and carbon oxide by-products (particularly CO) as well as to maximize hydrogen production. In the water-gas shift reaction carbon monoxide reacts with steam to produce carbon dioxide and additional hydrogen. This is often done in two steps. The high temperature shift (HTS) reaction usually runs at about 350° C. and reduces CO levels to about 1%–2%. The low temperature shift (LTS) reaction runs at about 200° C. and reduces the amount of CO down to about 0.1%–0.2%. In both cases, ideally the reaction is run in an excess of steam and at the lowest temperature possible to achieve the target conversion. Conventional iron/chromium-containing HTS catalysts are inactive below about 300° C. and copper/zinc-containing LTS catalysts lose the activity above about 250° C. Both the HTS and LTS catalysts require in-situ reduction treatments and are extremely air sensitive. All currently available LTS catalysts are either pyrophoric or have a relatively low activity. Some of them are based on expensive precious metals such as Pt, Pd, Rh. The pyrophoric nature of LTS catalysts contributes to an unacceptably rapid deactivation rate.
In the preparation of hydrogen for fuel cells, the WGS reaction zone can be the largest component of the fuel processor affecting its size, weight and performance factors such as its start-up time. Therefore, a WGS catalyst is needed which is air stable, low cost, and has high activity. In addition a WGS method that can operate over a wider temperature window without deactivation is needed. The present invention provides such a catalyst and method.